A polymer electrolyte fuel cell is characterized by using a proton conductive polymer electrolyte membrane, and is a device that provides electromotive force by electrochemically reacting a fuel gas, such as hydrogen, with an oxidizing gas, such as oxygen. The polymer electrolyte fuel cell can be utilized as a private power generator, or a power generator for a moving body, such as an automobile.
Such a polymer electrolyte fuel cell has a polymer electrolyte membrane that selectively conducts hydrogen ions (protons). In addition, the fuel cell has two gas diffusion electrodes and has a structure as described below. The gas diffusion electrode has a catalyst layer that contains, as a main component, a carbon powder that supports a noble metal-based catalyst, and has a gas diffusion electrode substrate. Each of the gas diffusion electrodes is joined to the surface of the polymer electrolyte membrane with the catalyst layer facing inward.
An assembly including such a polymer electrolyte membrane and two gas diffusion electrodes is referred to as a membrane electrode assembly (MEA). In addition, on both outer sides of the MEA, separators are installed in which gas flow paths are formed in order to feed a fuel gas and an oxidizing gas and to discharge produced gases and excessive gases.
A gas diffusion electrode substrate needs mechanical strength because the gas diffusion electrode substrate is fastened by a load of several MPa by a separator in order to reduce electric contact resistance and suppress the leakage of a fuel gas or an oxidizing gas fed from the separator to the outside of a fuel cell.
In addition, since the gas diffusion electrode substrate needs to mainly have the following three functions, the gas diffusion electrode substrate is usually a porous electrode substrate having a porous structure. The first function required of the gas diffusion electrode substrate is the function of uniformly feeding the fuel gas or the oxidizing gas from the gas flow path formed in the separator, which is located on the outer side of the gas diffusion electrode substrate, to a noble metal-based catalyst in the catalyst layer. The second function is the function of discharging water produced by a reaction in the catalyst layer. The third function is the function of conducting electrons necessary for the reaction in the catalyst layer or electrons produced by the reaction in the catalyst layer to the separator. Generally, what is considered to be effective in realizing these functions is that the gas diffusion electrode substrate is a carbonaceous material.
Conventionally, in order to increase mechanical strength of the substrate, short carbon fibers were formed to a paper and bound one another by using organic polymers, and then this paper is firing at a high temperature to carbonize the organic polymers and to produce a porous electrode substrate which is composed of carbon/carbon composites in paper shape. However, problems of this technique are that the production process is complicated and production costs are high. In addition, in order to reduce costs, a porous electrode substrate is proposed which is obtained by forming a paper from oxidized short fibers, and then firing the paper at high temperature to carbonize the oxidized short fibers. However, since the oxidized short fibers shrink during firing, problems of the electrode substrate are its dimensional stability and surface precision.
Patent Literature 1 discloses a porous carbon electrode substrate for a fuel cell characterized by having a thickness of 0.05 to 0.5 mm, a bulk density of 0.3 to 0.8 g/cm3, and a bending strength of 10 MPa or more and a deflection in bending of 1.5 mm or more in a three-point bending test under the conditions of a strain rate of 10 mm/min, a distance between support points of 2 cm, and a test piece width of 1 cm.
Patent Literature 2 discloses a carbon fiber sheet having a thickness of 0.15 to 1.0 mm, a bulk density of 0.15 to 0.45 g/cm3, a carbon fiber content of 95% by mass or more, a compressive deformation rate of 10 to 35%, an electric resistance value of 6 mΩ or less, and a degree of drape of 5 to 70 g.
Patent Literature 3 describes a mat that includes a plurality of carbon fibers; and a gas diffusion layer for a fuel cell that is obtained by incorporating a plurality of acrylic pulp fibers into the carbon fiber mat and then by curing and carbonizing them.